Verizon has its own exclusive version of the OnePlus 8, the "OnePlus 8 5G UW", which was specially developed for its network and offers additional mmWave radio functions to support Verizon's 5G network (which only operates in the mmWave band becomes). However, as noted by the Reddit user / u / bud-dho, this means that the volume keys of the Verizon version are lower than the unlocked and T-Mobile models due to changes to the phone that are adapted to the additional wireless technology so that standard OnePlus 8 sleeves were won’t fit (via 9to5Google).
An analysis of the FCC documents for the two variants by PhoneArena makes the problem clear: The Verizon model has several additional mmWave modules, one of which is located directly where the volume switches are located. It probably won't affect all cases – those with cutouts for the volume buttons may only have larger cutouts, but it may take some time for the case manufacturers to assert themselves, and cases that cover the buttons completely will not work.
The problem is that most cases for OnePlus 8 on websites like Amazon were designed for the unlocked / T-Mobile hardware, not for the specific version of Verizon. If you want a case that fits your OnePlus 8 5G UW, you probably need to get it directly from Verizon, where cases tend to be more expensive and selection is more limited.
The moving buttons aren't the only difference from the Verizon model – it's also designed for IP68 water resistance (as is the T-Mobile version), an official name that the unlocked version lacks.